IMDs, e.g., pacemakers, cardioverters, and defibrillators, are adapted, i.e., configured, to be implanted within a patient's body and to generate therapeutic electrical stimulation, which can be applied to a patient's heart. Typically, an IMD includes signal generation circuitry for generating the therapeutic electrical stimulation, e.g., a therapeutic waveform or a shock, which is delivered to the heart via one or more body-implantable leads. Each of the leads includes one or more electrodes, which deliver the electrical stimulation from the IMD to the patient's heart and sense electrical signals that are output from the heart.
The IMD includes a microprocessor, which receives the electrical signals that are sensed from the patient's heart by the electrodes. The microprocessor also can induce the signal generation circuitry to generate the therapeutic electrical stimulation based on the electrical signals that are sensed from the heart. The IMD includes a memory, which is coupled to the microprocessor and stores instructions that control the operation of the IMD. Also, the IMD includes a battery, which is used to supply power to the IMD's electronics, and one or more capacitors, for example, aluminum electrolytic capacitors, which receive and store electrical charge from the battery. The electrical charge that is stored in the capacitors is used to create the therapeutic electrical stimulation that ultimately is delivered to the patient's heart.
An example of an IMD is an implantable cardioverter-defibrillator (“ICD”), which is capable of sensing when the patient's heart is experiencing particular forms of tachycardia that require cardioversion or defibrillation. For example, a typical ICD, in combination with its associated leads and electrodes, can sense when the atria or ventricles are fibrillating. After sensing the fibrillation, the ICD's microprocessor induces the signal generation circuitry to develop a high-voltage shock, e.g., a voltage between approximately 400 volts and approximately 1,000 volts, which is applied to the chamber of the heart that is fibrillating via the leads and electrodes.
During this process, electrical energy output from the battery initially is stored in the capacitor(s), and then is applied to the patient's heart. When the high-voltage electrical stimulation is delivered to the heart, it depolarizes the heart's cells in the chamber so that the cells can repolarize and function in a normal fashion. Accordingly, it is desirable that an ICD develops and provides the electrical stimulation very quickly following the detection of such a cardiac event.
A design constraint related to IMDs in general, and ICDs in particular, is that the amount of electrical charge that can be output from the battery is limited. Consequently, it is desirable to conserve the battery's electrical charge as much as possible because replacement of an IMD's battery involves an invasive surgical procedure. In an effort to conserve the battery's limited electrical charge, the IMD's capacitors are left uncharged unless the IMD's microprocessor determines that a cardiac event has been detected. However, leaving the capacitors uncharged for extended periods of time, e.g., for greater than approximately one week to four weeks, can result in degradation of the capacitors.
For example, a typical high-voltage capacitor that is used in an ICD has plates that are separated by an oxide dielectric material, which will degrade over time in the absence of a voltage being applied across the plates. If the capacitor's dielectric material has degraded, a subsequent charging of the capacitor can result in a considerable leakage current occurring between the capacitor's plates. The leakage current can prolong the time that it takes to charge the capacitor, and thereby delay the delivery of the therapeutic electrical stimulation to the patient's heart. A delay in the delivery of the therapeutic electrical stimulation to the patient's heart can result in disastrous consequences for the patient. Also, the larger the value of leakage current, the larger the amount of energy that must be expended to charge the capacitor to the desired charge level, and thus, can result in an increase in the consumption of battery power and a decrease in the longevity of the IMD.
A method that currently is used to deal with the problem of dielectric material degradation is to have the IMD periodically charge the capacitors, e.g., once every one to three months, and hold the capacitors at a predetermined high-voltage level for a period of time, e.g., a minute or so, before discharging the capacitors through a load internal to the IMD even if no cardiac event has been detected. This periodic charging of the capacitors is called “reforming the capacitors,” and typically results in a rebuilding of the oxide layers within the dielectric material, which enables the capacitors to charge faster.
While the reformation process has the effect of reducing the degradation of the capacitor dielectric material, it does drain the battery, and thus, reduces the battery's longevity. Typically, the amount of charge that is required from the battery to reform an IMD's capacitor progressively increases over time. Thus, depending upon the age of the dielectric material, reforming the capacitors may result in the capacitors being charged before the capacitor's dielectric material has degraded to a point where the leakage current would present a problem. Consequently, while periodically reforming the capacitor may reduce degradation of the dielectric material and the associated leakage current, it accomplishes these goals at a significant cost in battery charge and IMD longevity. Finally, reforming the capacitor every few months does not adequately keep the capacitor at optimal performance because the time constant of degradation is on the order of a week.
It should, therefore, be appreciated that there is a need for an apparatus and a method for reforming IMD capacitors so that the leakage current is maintained within acceptable tolerances without requiring an excessive expenditure of battery power. The present invention satisfies this need as discussed below.